US3758805A

US3758805A - A high intensity light source

Info

Publication number: US3758805A
Application number: US00192857A
Authority: US
Inventors: M Geller; D Altman; G Barstow
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1971-10-27
Filing date: 1971-10-27
Publication date: 1973-09-11
Anticipated expiration: 1990-09-11

Abstract

A high intensity light source produces the principal portion of its light energy output substantially within the spectral range of 4,600 A to 5,000 A. A sealed envelope of material substantially transparent to light energy within the aforementioned range contains electrically conductive electrodes, a suitable starter gas, preferably of an inert character, and also an amount of elemental metal. The electrodes have external terminals which, upon connection to a suitable source of electrical energy, generate an initial electrical discharge producing sufficient heat to cause sublimination of the metal so that it is converted to its vapor state. The metal is chosen to have a sufficiently high vapor pressure so that it will sublimate in response to a temperature of the order of approximately 800*C. One such metal is zinc, for example. Moreover, the amount of elemental metal contained within the envelope is such as to develop a sufficient pressure within the envelope for producing the principal light energy output within the desired stated spectral range. In the preferred embodiment the starter gas is an inert gas and the spacing between the electrodes is a relatively short gap ensuring an extremely high spectral radiance.

Description

[ 1 Sept. ll, I973 A HIGH INTENSITY LIGHT SOURCE [75] Inventors: Myer Geller; Daniel E. Altman;

Glidden J. Barstow, all of San Diego,

Calif. 92106 57 ABSTRACT A high intensity light source produces the principal portion of its light energy output substantially within the spectral range of 4,600 A to 5,000 A. A sealed en- [73] Assignee; The United States of Ameri a a velope of material substantially transparent to light enrepresented by the Se retary of the ergy within the aforementioned range contains electri- Navy, Washington, DC. cally conductive electrodes, a suitable starter gas, preferably of an inert character, and also an amount of ele- 2 Flled 1971 mental metal. The electrodes have external terminals [21] Appl. No.: 192,857 which, upon connection to a suitable source of electrical energy, generate an initial electrical discharge pro ducing sufficient heat to cause sublimination of the [2%] 31322565134227 metal so that it is converted to its vapor Stata The 5 metal is chosen to have a sufficiently high vapor pres- 1 le 0 sure so that it will sublimate in response to a temperature of the order of approximately 800C. One such [56] References C'ted metal is zinc, for example. Moreover, the amount of el- UNITED STATES PATENTS emental metal contained within the envelope is such as 3,363,134 1/1968 Johnson 313/225 to develop a sufficient pressure within the envelope for FOREIGN PATENTS OR APPLICATIONS producing the principal light energy output within the 10 desired stated spectral range. In the preferred embodi- 7,764 6/1939 Australia 313 225 mem the Starter gas is an inert gas and the Spacing tween the electrodes is a relatively short gap ensuring Primary Examiner-Roy Lake an extremel hi h s ectral radiance. Assistant Examiner-Darwin R. Hostetter y g p Att0rneyR. S. Sciascia-et al. 8 Claims 5 Drawing Figures TINTENSITY I I I l I I I I I I l I I I I I I I I 4600 4700 4800 4900 5000 5500 6000 6100 6200 6300 6400 PATENTED SEPI I I975 SHEU 1 OF 2 IOOM 200 M ATTENUATION COEFFICIENT WAVELENGTH 3.

FIG. 1

TI A3 VAPOR PRESSURE (TORR) IO ll IOOO-r E Q I2 I50- FIG. 2 IO 1 I I I i i MYER GELLER 200 600 I000 BY DANIEL E. ALTMAN TEMPERATURE 0 1 GLI%TEN J. BARSTOW FIG. 5 MF 1 A IIIGII INTENSITY LIGHT SOURCE CROSS REFERENCE TO RELATED APPLICATION The subject matter of the present invention is generally related to that of copending U.S. Pat. application, Ser. No. 158,330, titled Light Source for Use in Deep Ocean Water, filed June 30, 1971 in the names of Myer Geller, Daniel E. Altman, and Glidden J. Barstow.

BACKGROUND .OF THE INVENTION Underwateroptical systems such as are employed for viewing, surveillance, range gating, communication,

etc., are best implemented with light sources producing. high energy outputs generally in the blue portion of the light spectrum and more particularly between the spectral range of 4,600 A and 4,900 Awhere ocean waters impose minimum attenuation and thus pennit the greatest transmissivity of light energy.

Prior art light sources presently employed in deep ocean underwater optical systems include a frequency doubled yttrium aluminum, garnet, neodymium doped, YAG(ND) laser radiating at about 5,300 A. This type of source is, however; unfortunately relatively inefficient inasmuch as its peak energy outputs occur at wavelengths which are fairly close to the desired spectral range but are nonetheless not within the maximum transmissivity of ocean water which lies substantially between the wavelengths 4,600 A and 4,900 A. Moreover, this type of light source requires very careful control of both its operative temperature and the optical alignment of its doubler.

The argon ion laseris another type of prior art light .source which has been used in underwater optical systems. The argon ion laser has two very strong lines of peak energy output at approximately 4,880 A and 5,140 A. However, it is also relatively very inefficient because the principal portion of its output energy is not within the spectral range where highest transmissivity occurs in ocean waters. Additionally, in the present state of the art output powers of more than five watts are not readily available from such a device.

Pulsed xenon lasers have also been employed experimentally in underwater optical systems, producing two strong outputs at about 4,954 A and 5,005 A which are reasonably close to the spectral range of maximum transmissivity or minimum attenuation in deep ocean waters. However, pulsed xenon lasers are still in a state of development and have the disadvantage of being relatively inefficient in respect of the amount of input power required to produce a minimally usable amount of light output energy.

A number of non-coherent light sources' are also available such as the tungsten incadescent lamps and the mercury or thallium iodide vapor arc discharge lamps. Unfortunately, however, these type of prior art light sources develop most of their radiant energy outside the limited spectral range within which deep ocean water has the highest degree of transmissivity.

SUMMARY OF THE INVENTION The present invention comprises a highly desirable and efficient light source for underwater optical systems, producing its principal light energy output substantially within the spectral range of 4,600 A to 5,000 A where the" greatest degree of transmissivity may be realized. That spectral range (because of its maximum transmissivity and minimum attenuation of light energy through deep ocean water) is sometimes referred to as the water window."

In its preferred embodiment the present invention comprises a sealed envelope fabricated of material which is substantially transparent to light energy within the aforementioned spectral range such as high grade quartz, for example. Sealed within the'envelope are two electrodes which are preferably spaced and positioned relative to each other so as to provide a short arc discharge through an appropriate starter gas. The starter gas is preferably an inert gas such as xenon which will insure minimum deterioration of the electrodes due to sputtering, corrosion and other deleterious effects.

The envelope also contains an amount of an elemental metal chosen for the physical characteristics it exhibits under certain controlled conditions which will be explained more fully hereinafter.

The electrodes extend from within the sealed envelope to electrically conductive external terminals suitable for connection to a source of electrical energy. Upon the application of electrical energy to the external terminals, the starter gas within the sealed envelope ionizes and creates a short arc between the relatively closely spaced electrodes.

The elemental metal contained within the envelope is chosen from one of several-metals having a sufficiently high vapor pressure to cause sublimination into a vapor state in response to the heat generated by the initial discharge of the ionized starter gas between the closely spaced electrodes; most importantly, the elemental metal must possess energy levels such that strong transitions occur in the water window" and that the transitions dominate the output spectrum; the amount of such elemental metal within the envelope must be such that it will develop a pressure within the envelope for producing the principal light energy output of the assem'bly'of the light source within the desired spectral range, i.e., substantially the water window.

In accordance with the concept of the present invention, a metal such as zinc or cadmium may be chosen because of its high vapor pressure characteristics which will enable it to subliminate and be transformed to its vapor state at a temperature of the order of approximately 800C. Additionally, the amount of such chosen elemental metal contained within the sealed envelope is sufficient to develop a significant vapor pressure which may, for example, be of the order of one atmosphere in a typical embodiment, enabling the light source assembly of the present invention to produce principal light energy output which differs significantly from the usual principal light energy output of a vapor arc lampemploying the same metal but operating at relatively low vapor pressure. A low vapor pressure lamp employing the same elemental metal within a sealed envelope produces its principal light source at wavelengths outside the desired spectral range of water window." The concept of the present invention, when practiced under the prescribed high pressure conditions and employing a 'metal chosen for its physical characteristics as taught by the present invention develops a principal amount of light energy output falling well within the desired spectral range of water window."

Accordingly, it is a primary object of the present invention to provide an improved metal vapor arc light 3 source for generating its principal light energy output within the spectral range of wavelengths having the greatest transmissivity in ocean waters.

Another most important object of the present invention is to provide such a lamp which is operative at relatively modest temperatures.

Yet another important object of the present invention is to provide such a lamp wherein a metal is chosen which has a high vapor pressure enabling the metal to sublimate to its vapor state in response to the heat of the are developed by an inert starter gas.

A further object of the present invention is to provide such a light source which will produce a principal portion of its light output energy within the desired spectral range when operated at a temperature below that which will cause the devitrification of quartz.

Another object of the present invention is to provide a light source producing its principal light energy output within the designed spectral range operative in a short are configuration for producing extremely high spectral radiance or brightness per unit wavelength.

These and other features, objects, and advantages of the present invention will be better appreciated from an understanding of the operative principles of a preferred embodiment-as described hereinafter and as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a graphic illustration of the attenuation of different wavelengths .of light at several different depths of ocean water;

FIG. 2' is an illustration of a preferred embodiment of the present invention;

FIG. 3 is a graphical illustration of the variation in vapor pressure developed by the elemental metal zinc in its vapor state relative to temperature;

FIG. 4 is a graphical illustration of the spectrum of light energy output developed by a light source embodying the present invention and employing elemental zinc;

FIG. 5 is a graphical-illustration of the spectrum of light energy output developed by a light source embodying the present invention and employing elemental cadmium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 graphically illustrates the variation of attenuation of light ocean water at different depths and also at different wavelengths of light. In the illustration of FIG. 1 the attenuation of light at different depths of meters, 100 meters, 200 meters, and 2,700 meters is illustrated by a series of curves which encompass varying wavelengths generally from approximately 4,000 A to 6,000 A. It is evident that less attenuation of light generally occurs at greater depths. Additionally, however, it is also evident from the illustrations of FIG. 1 that minimum attenuation or greatest transmissivity of light is realized at all depths within the spectral range of approximately 4,500 A to 5,000 A.

Accordingly, light sources for use in ocean waters desirably produce the principal portion of their light energy output within the range of greatest transmissivity which may be referred to as the water window." It is also desirable that a light source for use in an ocean environment be as simple in its concept and operation as 4" is possibleand consistent with the requirement that it be efficient in developing its light energy output within the desired spectral range to a high degree of spectral radiance.

FIG. 2 illustrates a preferred embodiment of the present invention which includes a sealed envelope l0 fabricated of a material which is substantially transparent to light energy within the desired spectral range, such as quartz, for example, and having two electrodes 11 and 12 sealed within its interior. The electrically conductive electrodes 11 and 12 have external terminals 13 and 14 adapted for connection to a source of electrical energy.

Within the interior of the sealed envelope 10 there is included a suitable starter gas preferably of a inert character, such as argon or xenon, and also an amount of a metal in its elemental form. The metal 'may be supported on the interior walls of the sealed envelope. The metal is chosen for a relatively high vapor pressure characteristic such as will enable the metal to be sublimated to its vapor state in response to the heat developed by the initial discharge of the conductive are between the two electrodes within the envelope upon the application of a suitable source of electrical energy to their external terminals.

As is well known in the art, the starter gas becomes ionized and forms a conductive arc to start the operation of the light source. The heat of the arc causes the subsequent sublimation of the elemental metal which changes directly to its vapor state.

Additionally, the metal must have strong electronic radiation transitions in the 4,600 A 5,000 A region. The concept of the present invention teaches that the metal must be chosen to have certain required characteristics. .For example, the energy level structure of atomic neutral zinc exhibits transitions of interest including three principal lines where transitions are of the type 5 S 4P. The atomic parameters of these three lines are:

4680.1 A has. gA 5.8 X 10 sec 4722.2 A has gA =15 10 4810.5 A has gA =21 X 10 When a starter gas such as argon or xenon is used, the 5 8 level is lower in energy than the lowest excited state of either argon or xenon. Thus, as the zinc metal evaporates and enters the discharge, the radiation from this multiplet dominates and the spectrum of the starter gas essentially disappears.

The amount of elemental metal which is sealed withinthe envelope 10 is carefully chosen so that a constant state of vapor saturation will prevail and a relatively high vapor pressure will be developed within the sealed interior of the light source when it reaches its fully operative state. FIG. 3 illustrates the variation in vapor pressure of zinc in its elemental form responsive to a range of temperatures under vapor saturation conditions. From the illustration of FIG. 3 it can be seen that when elemental zinc is heated to a temperature of approximately 900C it will develop a vapor pressure of nearly one atmosphere. In accordance with the concept of the present invention, a sufficient amount of the elemental metal is sealed within the interior of the light source for developing a relatively high pressure at a fairly modest temperature, with the result that the prin cipal portion of the light energy output developed and generated is not only distinctly different from the spectral range of light energy output developed in a metal are vapor lamp employing zinc at a relatively low pressure, but also desirably falls within the range of approximately 4,600 A to 5,000'A which has been referred to as the water window."

Moreover, the electrodes iland 12 are preferably spaced in a short arc configuration providing high spectral radiance, i.e., brightness per unit wavelength. A preferred embodiment of the present invention substantially as illustrated in FIG. 2 with a power input of 800 watts, and employing the short arc" configuration having electrodes spaced at- 0.10 inches, emitted 13 watts within the desired spectral range comprising the water window. Thus, the light'source of the present invention provides an extremely high spectral radiance and produces the principal portion of its light energy output within the desired spectral range.

FIG. 4 is a graphical illustration of actual test data developed from the operation of a light source embodying the concepts of the present invention, using argon as the starter gas, and zinc as the elemental metal. It will be seen that extremely strong peaks of light energy output are developed at approximately 4,680 A, 4,720 A, and 4,810 A. A fourth strong output of energy is developed at approximately 6,360 A which is outside the most desirable spectral range. However, the greater portion by far of the light energy output of the light source constructed and operated in accordance with the concept and teaching of the present invention is clearly within the desired spectral range comprising the water window. The conventional low pressure zinc arc vapor lamp produces distinctly different spectral outputs spread over an extremely broad spectral range so that its employment as an acceptably efficient underwater light source is for all practical purposes almost wholly ineffective. The highly desirable results produced by the concept of the present invention is achieved through the use of an amount of elemental metal in sufficient quantity to ensure saturation condi-' tions at the operative temperatures and develop relatively high vapor pressure which brings about the principal output of light energy within the desired spectral range.

Another suitable elemental metal which may be emtemperatures are well below the temperatures at which quartz will devitrify, exhibiting undesirable deterioration effects'such as clouding, etc. Many prior art known' lamps which attempt to achieve the same ob- 5 jects and purposesas the present invention are required ployed in accordance with the concept of the present invention is cadmium and the same general teachings are equally applicable in the use of cadmium as-in the previously described employment of zinc.

FIG. 5 graphically-illustrates a light energy output of a lamp fabricated and operated in accordance with the concepts of the present invention and employing cadmium as the elemental metal. In the illustration of FIG. 5 it may readily been seen that three principal peaks of light energy output are developed at approximately 4,675 A, 4,797 A, and 5,085 A, with another single peak of high energy output developed in the general range of approximately 6,450 A. Thus, the use of cadmium in a light source embodying the present invention tive efficiency and the production of the principal portion of its useful energy output within the desired spectral range when the light source is operated at a relatively modest temperature. Desirably, these operative to operate at significantly higher temperatures, sharply limiting the life of quartz envelopes, or requiring the employment ofdifferent materials other than quartz which are not subject to deleterious effects at such high temperatures.

Another and most important advantage of the present invention is that in its preferred embodiment it is operative in a short arc" configuration which provides a concentration of light energy output to develop extremely high spectral radiance not readily achievable with known prior art light-sources capable of producing significant amounts of output energy within the desired spectral range and operating in the continuous mode.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

L'A high intensity light source producing its principal light energy output substantially within a predetermined spectral range comprising:

a sealed envelope of material substantially transparent to light energy within said spectral range;

electrically conductive electrodes communicating with the interior of said envelope, forming a short gap therebetween, and having external terminals for connection to a source of electrical energy, said short gap being dimensioned and configured relative to the current capacity of said source of electrical energy for producing a high .power density am;

a starter gas contained within said sealed envelope for generating an initial electrical discharge between said electrodes upon the connection of a source of electrical energy to said terminals; and

a metal contained within said envelope;

said metal having sufficiently high vapor pressure to cause its sublimation in response to the heat generated by said initial discharge, and being present in an amount developing a pressure within said envelope of a magnitude for sustaining said high power density are and generating its principal light energy output within the spectral range of 4,600A to 5,000A.

2. A light source as claimed in claim 1 wherein said metal is initially supported on the inner walls of said envelope.

3. A light source as claimed in claim 1 wherein said metal is zinc.

4. A light source as claimed in claim 1 wherein said metal is cadmium.

5. A light source as claimed in claim 1 wherein said starter gas is an inert gas.

6. A light source as claimed in claim 1 wherein said starter gas is argon. I

7. A light source as claimed in claim 1 wherein said starter gas is xenon.

8. A light source as claimed in claim 1 wherein said envelope is fabricated of quartz material.

t! t t 4' 4

Claims

2. A light source as claimed in claim 1 wherein said metal is initially supported on the inner walls of said envelope.
3. A light source as claimed in claim 1 wherein said metal is zinc.
4. A light source as claimed in claim 1 wherein said metal is cadmium.
5. A light source as claimed in claim 1 wherein said starter gas is an inert gas.
6. A light source as claimed in claim 1 wherein said starter gas is argon.
7. A light source as claimed in claim 1 wherein said starter gas is xenon.
8. A light source as claimed in claim 1 wherein said envelope is fabricated of quartz material.